1. Field of the Invention
The invention relates in general to an apparatus and method for data signal conversion, and more particularly to an apparatus and method for data signal scattering conversion.
2. Description of the Related Art
A display apparatus is used as means of communication between humans and machines. Two kinds of display apparatus, the cathode ray tube (CRT) display and the liquid crystal display (LCD), are available in the market. For CRT displays, since their technology and manufacture are well developed, their cost is relatively low even for providing high quality color images, so that they are widely used. However, CRT displays are large in size and emit high levels of radiation. On the other hand, LCDs can be made more compact, with low emissions of radiation. Therefore, LCDs, such as thin-film transistor liquid crystal displays (TFT-LCDs), are being substituted for CRT displays.
Referring to the block diagram of FIG. 1, a TFT-LCD 100 is illustrated to include a display panel 110, a data driver 120, and a scanning driver 130. Display panel 110 includes a plurality of pixel units P configured to form an m by n array, wherein each pixel unit P includes a thin film transistor and a liquid crystal device (not shown). For the pixel units in each column, source terminals of the thin film transistors are electronically coupled, forming m data lines 122 stretched out within the display panel 110. Likewise, for the pixel units in a row, gate terminals of the thin film transistors are electronically coupled, forming n scan lines 132 stretched out within display panel 110. Data driver 120 includes a front-end processor 126 and a data signal converter 128, and is used to receive digital image data D and output analog data signals A. Front-end processor 126 is employed to receive the digital image data D and output digital data signals D′. Data signal converter 128 is coupled to front-end processor 126 and display panel 110, and is used to receive the digital data signals D′, perform digital-to-analog (D/A) conversion of the digital data signals D′ so as to produce analog data signals A, and then output them to display panel 110. Scanning driver 130 is coupled to scan lines 132 and is to receive a horizontal synchronization (HSYNC) signal and a vertical synchronization (VSYNC) signal.
According to the VSYNC signal, scanning driver 130 sequentially selects each of the scan lines 132 (scan line 132(k), k=1 to n), so as to turn on all the thin film transistors of the selected scan line. When all the thin film transistors of the scan line 132(k) are turned on, the analog data signals A from data driver 120 are applied to the liquid crystal devices of the scan line 132(k) through source and drain terminals of the thin film transistors of scan line 132(k) for control of the gray levels of the liquid crystal devices. In this manner, data driver 120 controls the gray levels of the liquid crystal devices according to the analog data signals A. When scanning driver 130 receives the VSYNC signal, scanning driver 130 re-starts to turn the scan lines 132 sequentially on at a time from the first (k=1) to the last (k=n). Generally, the time period between two successive HSYNC signals is denoted as a horizontal scanning time, while the time period between two successive VSYNC signals is denoted as a vertical scanning time. For displaying a frame, it takes one horizontal scanning time to complete one horizontal line of the frame, and takes one vertical scanning time to complete the entire frame.
In practice, the liquid crystal device is easily damaged when voltages of the same polarity are continuously applied to the liquid crystal devices. Accordingly, data driver 120 may apply polarity inversions to avoid such damage on liquid crystal device. Polarity inversion such as dot inversion or column inversion is to alternately output positive and negative voltages to the liquid crystal devices.
FIG. 2 illustrates details of the data signal converter 128 shown in FIG. 1. Data signal converter 128 includes a digital-to-analog (D/A) converter. The converter 128 receives the digital data signals D′, performs polarity inversion of the digital data signals D′, and outputs the analog data signals A. The converter 128 includes m demultiplexers 202, m multiplexers 204, m+1 digital-to-analog conversion devices 206, and m+1 output buffers 210. For instance, demultiplexer 202(i) is used to receive digital data signal D′(i) and to output digital data signal D′(i) to either D/A conversion device 206(i) or 206(i+1) according to the polarity inversion method. If i is an odd number, D/A conversion device 206(i) is to output converted data signal S(i) with positive polarity. If i is an even number, D/A conversion device 206(i) is to output converted data signal S(i) with negative polarity. Output buffer 210(i) is used to receive converted data signal S(i), output buffered data signal S′(i), and feed buffered data signal S′(i) into multiplexers 204(i−1) and 204(i). Multiplexer 204(i) is employed to receive buffered data signals S′(i) and S′(i+1) from output buffers 210(i) and 210(i+1) respectively, and to selectively output digital data signal A(i) according to demultiplexer 202(i), where A(i) is either S′(i) or S′(i+1). For example, demultiplexer 202(i) outputs digital data signal D′(i) to D/A conversion device 206(i+1) so that multiplexer 204(i) outputs S′(i+1) as analog data signal A(i). In this way, by using demultiplexers 202 and multiplexers 204, the polarities of individual analog data signals A can be changed according to the polarity inversion method, and analog data signals A after polarity inversion are associated with appropriate data lines 122. If demultiplexer 202(i) and multiplexer 204(i) are to change the polarity of analog data signal A(i) according to the HSYNC signal, the dot inversion is therefore achieved. If demultiplexer 202(i) and multiplexer 204(i) are to change the polarity of analog data signal A(i) according to the VSYNC signal, the effect of column inversion is achieved.
However, the analog data signals from the data driver may have different offset voltages, which correspond to gray lines displayed on the LCD and affects the uniformity of the brightness of each pixel displayed. Generally, the data driver produces the offset voltages due to variations of output voltage levels of the operational amplifiers in output buffers 210. The offset of output voltage level of an operational amplifier is commonly in the range of 50 mV to 60 mV, while offset voltage tolerated by the LCD is within 10 mV. If the offset in the output of the operational amplifier exceeds the tolerance by too much, the associated liquid crystal device of display panel 110 may become a pixel unit with undesired deep color or light color.
FIG. 3A illustrates a frame 300 displayed by TFT-LCD 100, wherein each rectangle represents a pixel unit and the output buffer 210(j) has an output offset voltage. Output buffer 210(j) outputs analog data signal A(j) to data line 122(j) so that the pixel units controlled by data line 122(j) displays a light gray line. FIG. 3B shows a graph of gray level intensity versus data line, wherein the data lines shown in FIG. 3A are indicated along the X-axis, while the gray level intensities perceived by humans are measured along the Y-axis, and wherein data driver 120 outputs image data by the column inversion method. Since light stimulus will be integrated in the human visual system, the gray line corresponding to output buffers 210(j) occurs as shown in FIG. 3A.
FIG. 4A illustrates another frame 400 displayed by the TFT-LCD 100, wherein each rectangle represents one pixel unit and data driver 120 outputs image data by the dot inversion. Output buffer 210(j) has an output offset voltage. Thus, when output buffer 210(j) outputs analog data signal A(j) to data line 122(j) or outputs analog data signal A(j−1) to data line 122(j−1), the pixel units controlled by data lines 122(j) and 122(j−1) displays light gray points indicated in FIG. 4A. By the integration effect of the human visual system described above, these light gray points are actually perceived by humans as light gray lines displayed on the LCD panel, as shown in FIG. 4B. The X-axis in FIG. 4B indicates the data lines of FIG. 4A while the Y-axis indicates the gray level intensities perceived by human eyes.
For resolving the problem of degradation of the uniformity of display brightness due to the variation in output signal level, one way is to improve the output precision of the operational amplifiers to be used. However, this solution greatly increases the difficulty in the design and manufacture of LCDs.